The present invention relates to a new and improved construction of an excess pressure steam turbine having a constant pressure regulation stage which is arranged forwardly of the reaction blading of the turbine and consists of a regulation impeller and a nozzle scroll or rim of nozzles which can be throttled in sectors.
In order to improve the partial load efficiency there is usually arranged forwardly of the reaction blading of a reaction steam turbine a reaction stage designed as a constant pressure stage. Here the load regulation is accomplished by closing or throttling individual regulation valves operatively associated with a respective sector of the rim or row of nozzles. This partial fluid impingement of the reaction stage results in pronounced irregularities in the flow at the outlet side of the regulation impeller, and thus, results in additional losses in the subsequent stages of the reaction portion or section of the turbine. However, these losses decrease with decreasing degree of reaction of such stages. In the case of small reaction degrees which differ only slightly from null they are appreciably smaller than for the conventionally designed reaction degrees of about 0.5.
The heretofore constructed steam turbines designed for intermittent partial load operation predominantly belonged to the lower and intermediate class of power turbines. In such turbines the diameter of the regulation impeller can be designed appreciably larger than the diameter of the rotor drum forwardly of the first reaction stage without endangering its strength due to centrifugal forces. For this reason there is beneficially obtained between the exit plane of the regulation impeller and the inlet plane of the first reaction stage so much space that the flow following the regulation impeller, with partial fluid impingement, until entry at the reaction portion can be extensively uniformly distributed over the entire cross-section of the flow channel, so that the losses owing to flow irregularities in the first reaction stage can be maintained within narrow limits.
On the other hand, in the case of steam turbines designed for maximum power outputs it is not possible, because of the too large centrifugal forces, to design a possibly provided impeller wheel that much larger in relation to the reaction portion so that the space between the impeller wheel and the first reaction stage is adequate for rendering more uniform the flow over the entire circumference of the flow channel. Such regulation is therefore associated with appreciable losses throughout the partial load range. In the extreme case the impeller wheel only can be designed to have the same diameter as the turbine blading of the first reaction stage, so that during partial impingement of the turbine blading the flow is quite incomplete and the efficiency is appreciably impaired.
This does not play any decisive role as long as it is possible, with acceptable energy costs to permit such turbines to operate continuously with practically constant base load. However, at the present time, owing to the enormously increased energy costs, there now exists to an increasing degree the tendency of operating in a throttled mode turbo-generator units designed for high outputs because of the uneconomical operation during the weak load times. This would be economically feasible with a constant pressure regulation if there were available in the case of smaller turbines sufficient space between the regulation stage and the reaction portion, in order to obtain a homogenous flow forwardly of the reaction portion. Due to the practically constant diameter of the regulation stage and the first reaction stage such would however require a long channel devoid of blading between the regulation stage and the reaction portion or section of the turbine, with correspondingly greater material and machining costs.